Chromium diffusion process and article



United States Patent 3,253,942 CHROMIUM DIFFUSION PROCESS AND ARTICLEKarl Bungardt, Krefeld, and Gottfried Becker, Dusseldorf, Germany,assignors to Howe Sound Company, New York, N.Y., a corporation ofDelaware No Drawing. Filed Dec. 15, 1961, Ser. No. 159,740 3 Claims.(Cl. 117-50) This invention relates to the provision of chromiumdiffused surfaces on various components. In particular it is the purposeof this invention to provide chromium diffused surfaces on articleswhich are to be subjected to .extreme conditions including exposure tohigh temperatures and to corrosive environments. Various alloys,ineluding steels and high temperature alloys, have found wide use inapplications where stress is experienced under adverse environmentalconditions. For example, various steels in view of their high strengthare desirably employed in corrosive atmospheres. Similarly, certainiron, nickel and cobalt base alloys containing chromium and otherelements are known to have suitable strength at extremely hightemperatures.

There is a constant demand for materials which can withstand corrosionin more severe environments and which also can hold up under even highertemperature conditions. In the production of aircraft, rockets,missiles, space vehicles and the like, extreme corrosive atmospheres arerealized in areas near the combustion chambers and ever increasingstrength-temperature combinations are prevalent. It is also well knownthat various industrial facilities are subject to extreme corrosiveconditions requiring metals which will not deteriorate and which willretain their physical properties over extended periods.

It has been proposed that steels and various other alloys could besurface treated in a manner such that their oxidation resistance tocorrosion would be increased and, accordingly, the properties of thealloys would be relatively stable under various extreme conditions.Siliconizing, chromizing and aluminizing techniques have been attemptedas surface treatments, but the results have not been entirelysatisfactory. Surface layers produced by known techniques have notprovided a sufficiently impervious barrier to the corrosive mediumswhereby corrosion will eventually proceed as if there were no protectivebarrier. In addition, prior surface layers have not adequately withstoodthermal shock, and these layers have exhibited a tendency to fall awaywhen subjected to rapid and severe temperature differentials. I

It is an object of this invention to provide a'method for improving theoxidation resistance, the resistance to intergranular corrosion and toimprove in general the elevated temperature characteristics of alloysincluding carbon and low alloy steels and the various iron, cobalt andnickel base alloys containing chromium and other alloying ingredients.

It is an additional object of this invention to provide an improvedsurface treatment for alloys of the above described types whereby thedesired stability under adverse environmental conditions can beachieved.

It is a more specific object of this invention to provide a chromizingtechnique whereby various alloys can be provided with improved oxidationresistance, resistance to corrosion and improved elevated temperaturecharacteristics.

These and other objects of this invention will appear hereinafter, andit will be understood that the specific embodiments hereinafter setforth are provided for purposes of illustration and not by way oflimitation.

It has been previously known to enrich the surfaces of iron and steelparts with chromium by diffusion techniques in order to providecorrosion resistance. In the past such ice parts were heated totemperatures between 1000 C. and 1100 C. in the presence of a chromiumcompound and in the absence of oxidizing gases. Thus, chromium orferrochromium pieces, along wtih aluminum oxide, porcelain sillimaniteand a halogen salt such as ammonium chloride, have been placed in aretort along with parts to be treated in order to diifuse chromium intothe surfaces of the parts.

Although the known methods have been practiced only to a limited extentand, in most cases, on alloy steels, it has recently become desirable toprovide chromium on parts composed of higher alloy steels and nickel andco balt alloys. Thus, it has been found that various high temperaturealloys are strongly attacked at temperatures as low as 700 C. whendisposed in hot combustion atmospheres. Similarly, scaling of thecomponents has been detected at temperatures as low as 1000 C. in theabsence of strongly corrosive atmospheres. It will be obvious that theexpensive high alloyed materials are severely damaged and become uselessafter a short period of use once they begin to scale or corrode.Conventional chromizing techniques have been employed on such alloys.However, these techniques have only delayed corrosive attack and havenot substantially reduced scaling resistance.

It has been found, in accordance with this invention, that high alloyedsteels and nickel and cobalt base alloys can be successfully providedwith a chromized 'layer whereby the articles will effectively resistcorrosion and scaling. It has additionally been found that theprocedures of this invention can be profitably employed in place ofconventional techniques, even in the case of carbon and low alloysteels. In connection with the procedures of this invention, it has beendetermined that the chromium diffusion treatment is only effective ifthe surfaces of the articles to be chromized are prepared in a distinctmanner prior to the chromizing operation. Specifically, it has beenfound that the roughness of the surfaces of the articles to be chromizedmust be kept below 98 micro-inches and prefrably below 79 micro-inches.

After preparation of the surfaces of the articles, a specific chromizingtechnique must be employed in order to provide the results of thisinvention. It must be specifically noted that conventional processing,if applied to articles having the above noted prepared surfaces, willnot provide the results of this invention.

In the'specific chromizing process employed, a chromium powder is addedin an amount of about 20% to the chromizing compound. The chromiumpowder is not only used in this relatively small amount, but it is alsocharacterized by a particle size of less than 5 microns.

In a further distinction of the process of this invention, the chromiumpowder is employed along with inactive oxides such as aluminum oxide ormagnesium oxide. The particles of the inactive oxides are preferablyretained between the range of 50 and 100 microns. It is important thatthe inactive oxides or oxide mixtures do not influence the course of theprocess. It has been suggested that porous ceramic pieces be employed inthe chromizing mixture which will serve to retain chromium values andwhich will serve to retain a consistent concentration within thetreating chambers. However, these ceramic pieces or other materialsincluding porcelain and sillimanite are specifically avoided in thepresent process since these materials have been found to increase thesurface roughness of the articles chromized.

The use of a halogen salt along with the chromizing compound is feasiblein accordance with this invention. However, the amount of the saltpreferred is unusually small and should-only be about 0.01%. Thechromizing temperatures and treatment periods are conventional.

However, the highest practical treating temperature should be employed,since this will tend to increase the upper temperature limit of use.

The chromized articles produced in accordance with the described processare characterized by a final surface finish of less than-'98micro-inches and, in the preferred situation, below 79 micro-inches. Thearticles produced have an extremely great resistance to corrosion atnormal temperatures, and this resistance is retained at temperaturelimits beyond what heretofore has been considered 1 an upper limit. Inaddition, the chromized layers have proven exceedingly tough and,accordingly, there is no danger of the layers chipping off when thearticles are subjected to thermal shock. The process of this inventionhas been found satisfactory in connection with extremely low alloyediron materials. For example, soft iron with a carbon content of lessthan .05% exhibited a rough surface when chromized by conventionaltechniques. In accordance with this invention, the desired smoothdiffusion surface can be obtained. The value of the disclosed conceptsis even more apparent when considering the ability of high alloyedmaterials to withstand scaling, corrosion and thermal shock. Obviously,turbine vanes, combustion chamber parts and similar articles which aresubjected to severe conditions can be expected to perform moresatisfactorily when treated in accordance with this invention. Thefollowing examples will serve to further illustrate the value of thedisclosed chromizing process:

Example I An alloy of the following composition was employed in achromizing treatment:

Percent Ni 70 Cr 12 W Al 5 Mo 3.5 Ti, Nb, Ta 2.5 Fe r Balance Componentsprepared from the above composition were placed in a retort along with achromizing mixture consisting of chromium and the remainder aluminumoxide. The chromium particles exhibited a particle size of less than 5microns, while the aluminum oxide exhibited an average particle size ofabout 80 microns. The retort was heated to. 1100 C. whereby a chromizedlayer was produced in the components with the surface of the com- Fe ,1,=,,ga, Remainder Components composed of alloys of this composition, whentreated in accordance with the procedure of Example I, have exhibitedchromized layers having the characteristics of this invention.-Specifically, the chromized articles described in Examples I and IIhaving a final surface finish of 98 micro-inches and they reveal anexceptional ability to resist scaling at elevated temperatures and toresist the effects of corrosive atmospheres up to extreme temperatures.The chromized layers, in addition, prove to be tenacious and exceedinglytough whereby thermal shock conditions did not result in deteriorationof the component surfaces.

It will be understood that various modifications can be made in thechromium diffusion concepts disclosed which provide the characteristicsof this invention without departing from the spirit thereof,particularly as defined in the following claims.

We claim:

1 1. A process for the production of metal articles adapted to withstandsurface deterioration at elevated temperatures comprising the steps ofprocessing said articles to achieve a surface roughness of less than 98micro-inches and chromizing the articles by heating to a temperatureabove 1000 C. in a powder mixture of metallic chromium and inactiveoxides containing about 20% by weight of chromium and in which theaverage particle size of the v chromium is less than 5 microns and inwhich the inactive oxides are selected from the group consisting ofaluminum oxide and magnesium oxide and having an average particle sizewithin the range of 50 and microns. '2. A process according to claim 1wherein a halogen salt in an amount of about .01% is included in saidmixture.

3. A process for the production of metal articles adapted to withstandsurface deterioration at elevated temperatures comprising the steps ofprocessing said articles to achieve a surface roughness of less than 79micro-inches and chromizing the articles by heating the articles to atemperature above 1000 C. in a powder mixture containing about 20%chromium, about .0l% of a halogengt and inactive oxides and in which themetallic chromium has a particle size of less than 5 microns and whereinthe inactive oxides are selected from the group consisting of aluminumoxide and magnesium oxide and have a particle size between 50 and 100microns.

References Cited by the Examiner UNITED STATES PATENTS 1,853,369 4/1932Marshall 117 22 2,141,640 12/1938 Cooper 117 22 2,811,466 10/1957 Samuel11722 2,999,766 9/1961 Ashworth et al 117-50 X FOREIGN PATENTS 363,95412/1931 Great Britain.

RICHARD D. NEVIUS, Primary Examiner. R. S. KENDALL, Assistant Examiner.

1. A PROCESS FOR THE PRODUCTION OF METAL ARTICLES ADAPTED TO WITHSTAND SURFACE DETERIORATION AT ELEVATED TEMPERATURES COMPRISING THE STEPS OF PROCESSING SAID ARTICLES TO ACHIEVE A SURFACE ROUGHNESS OF LESS THAN 98 MICRO-INCHES AND CHROMIZING THE ARTICLES BY HEATING TO A TEMPERATURE ABOVE 1000*C. IN A POWDER MIXTURE OF METALLIC CHROMIUM AND INACTIVE OXIDES CONTAINING ABOUT 20% BY WEIGHT OF CHROMIUM AND IN WHICH THE AVERAGE PARTICLE SIZE OF THE CHROMIUM IS LESS THAN 5 MICRONS AND IN WHICH THE INACTIVE OXIDES ARE SELECTED FROM THE GROUP CONSISTING OF ALUMINUM OXIDE AND MAGNESIUM OXIDE AND HAVING AN AVERAGE PARTICLE SIZE WITHIN THE RANGE OF 50 AND 100 MICRONS. 